1. Field of the Invention
The present invention relates to a communication system and apparatus and, more particularly, to a communication system and apparatus for modulating a signal to be communicated into a chirp signal and executing a communication.
2. Related Background Art
Hitherto, an optical spatial communication for executing a communication by using an aerial propagation of an optical signal has been known. In the optical spatial communication, since it is easily influenced by a disturbance because of the characteristics of a transmission path as an open space, in general, the signal is subjected to a certain kind of modulation and, thereafter, the modulated signal is transmitted rather than the signal in its own form.
In the case of the optical spatial communication, when long distance transmission is executed in the outdoors or the like, the signal light is greatly attenuated in dependence on a meteorological condition such as rain or the like. Therefore, it is necessary to detect a very weak signal. In the case of using the conventional modulating system such as FSK or ASK, there are problems such that the communication distance is limited and the reliability is low.
On the other hand, as an optical spatial communication for a short distance, there has been considered a method of diffusing the signal light to widen a space range where the signal can be received instead of the one-to-one communication. However, even in such a case, since the signal light is very weakened, the conventional transmission system has a drawback such that it is also difficult to assure the reliability of the communication.
Therefore, the applicant of the present invention has proposed a technique for modulating an input signal into a chirp signal and executing the communication in U.S. patent application Ser. No. 368,840.
According to the technique disclosed in U.S. patent application Ser. No. 368,840, a binary digital signal of "1" and "0" is converted into a predetermined code and transmitted or received. As shown in FIG. 11A, a communication is executed by modulating the signal by use of a modulation system in which the presence and absence of a chirp signal cs are respectively made to correspond to a binary digital signal of "1" and "0" by a chirp conversion element 31. Or, as shown in FIG. 11B, a communication is performed by modulating the signal by use of a modulation system in which chirp signals cs.sub.1 and cs.sub.0 of different patterns are respectively made correspond to a binary digital signal of "1" and "0" by chirp conversion elements 31 and 41.
Waveshapes on the right side in FIGS. 11A and 11B show modulated signals which were formed by the above modulation systems. The chirp signals cs, cs.sub.1, and cs.sub.0 are obtained by expanding the frequency component of an input signal (square wave pulse) in the direction of a time base by the chirp conversion element 31 or 41 on the basis of different expansion characteristics, respectively.
Particularly, in FIG. 11B, the chirp signals cs.sub.1 and cs.sub.0 are respectively formed by the chirp conversion elements 31 and 41 and an output signal on the right side in the diagram is obtained by adding those chirp signals.
According to the above system, in the transmitting section, a carrier wave of a predetermined frequency is modulated by pulses which express a transmission digital signal and the chirp signal in which the spectrum component of the modulated signal was expanded in the time base direction is output, while in the receiving section, the spectrum of the chirp signal received is compressed in the time base direction and, thereafter, the compressed signal is detected and the digital data is reproduced. Therefore, there is obtained the same effect as if the transmission energy was increased when the chirp signal is compressed on the reception side, and the apparent S/N ratio can be improved. Therefore, even a very weak signal such that it is difficult to execute the detection and reproduction by the conventional FSK or PSK modulation system can be detected and reproduced and the communication reliability can be improved. Therefore, even in the system for diffusing the signal light such that a number of receiving sections can receive the signal light, the communication can be certainly executed as well as the long distance communication.
However, according to the system of FIG. 11A, the presence and absence of the chirp signal are made correspond to codes "1" and "0" of the digital signal. Therefore, there is an uncertain point in separation and discrimination of the codes "1" and "0". For instance, upon demodulation, there is a possibility such that the code "0" is erroneously decided to be the code "1" due to the influence by noise on the transmission path or the like.
To solve the above problem, the system as shown in FIG. 11B has been considered. However, according to such a system, as will be obvious from the diagram, the chirp conversion elements having different conversion patterns are necessary, while on the reception side as well, counter conversion elements which can detect chirp signals of a plurality of patterns are also obviously needed, so that there are problems such that the circuit construction is complicated, the circuit scale is large, and the costs are high as compared with those of the system of FIG. 11A.